In large endless non-metallic conveyor belts used to convey bulk material, there is a possibility of encountering a rip in the belt, for example, by a sharp object dropped thereon at the loading station. It is desirable promptly to detect such rips and, preferably, to shut down the conveyor belt upon such detection, thereby minimizing damage to the belt. One such conveyor belt rip detector is disclosed in U.S. Pat. No. 3,792,459. In such rip detector plural antennas, which may be single electrical conductors, are embedded in the belt transversely to its length at spaced-apart locations in the belt. An electrical signal is coupled by respective integral antennas as they pass a rip detector station and capacitively couple therewith a transmitter and a receiver respectively located near opposite edges of the belt. However, a broken antenna, for example at a place where the belt has been ripped, will not couple the transmitter signal through to the receiver, and a detector circuit, then, senses the same as an indication of the occurrence of a rip condition. The detector circuit in such patent is operative after the lapse of a predetermined time period, which corresponds to the time required for the passage of a given number of broken antennas past the rip detector station without the coupling of the transmitter signal to the receiver, to produce a distinguishable output that activates an alarm and/or deactivates the conveyor belt drive.
A particular disadvantage with the system of U.S. Pat. No. 3,792,459 is the inability to provide accurate rip detector function during acceleration and deceleration periods of the belt, which can be a critical time when rip monitoring is desirable, for by providing accuracy of detection during slow operation of the belt, the accuracy of detection during full speed belt operation is reduced. The undesirable inaccuracy of such prior rip detector due to the long waiting period to see whether after an antenna has been missed the next antenna is detected and/or at least to accommodate the largest spacing between adjacent antennas means that a rather long and expensive to repair rip may occur before the conveyor is shut down. Moreover, since such conveyor belts are known to stretch, for example on the order of about 20%, during useful life, the indicated waiting period must be adequately long to accommodate the lengthened belt, thus further reducing the accuracy of detection and promptness of shut-down.
Sometimes an antenna in a belt may become worn so as not to provide a reliable signal. For example, the antenna may be broken intermediate its ends, but sometimes the ends touch to couple a weak signal between the transmitter and receiver and sometimes the ends may not touch. Such antennas, then, are unreliable and may cause unnecessary nuisance shut-downs.
Several of the disadvantages encountered in prior art rip detector systems have been overcome by the apparatus and method disclosed in commonly assigned U.S. Pat. No. 4,228,513. In such rip detector system the location of each antenna in the belt is stored electronically as a function of belt position, for example, which is sensed by a tachometer. When an anticipated antenna is not sensed at a detector station, including a transmitter and receiver, such failure of detection may be sensed and used to cause shut-down of conveyor system. As a valid antenna passes the detector station, information concerning the actual location of the antenna is updated, thus providing compensation for belt stretching. Additionally, information concerning location of a worn or broken antenna where the belt integrity is satisfactory can be deleted from memory.
A signal detection circuit especially for rip detectors is disclosed in commonly assigned U.S. patent application Ser. No. 871,664, filed Jan. 23, 1978, now U.S. Pat. No. 4,229,735, issued Oct. 21, 1980. Such circuit avoids falsely responding to a noise signal as an indication of an antenna presence. Such circuit also provides wave shaping of a signal properly received from an antenna to optimize response of detector circuitry thereto.